PARIS, Nov 7 (Reuters) - President Emmanuel Macron's government on Tuesday postponed a long-held target to reduce the share of nuclear energy in France's electricity generation.

Environment Minister Nicolas Hulot told a news conference that it was not realistic to reduce the share of nuclear energy in the power mix to 50 percent by 2025 from 75 percent at present but he did not set a new deadline.

"We will reduce the share of nuclear in the mix as soon as possible," he said, adding that doing so in a hurry would increase France's CO2 emissions and could endanger the security of power supply and put jobs at risk.

He also said that the Fessenheim nuclear plant, France's oldest, would be closed during Macron's five-year term.

The previous government of Socialist Francois Hollande voted in 2015 an energy transition law which set a target of reducing the share of nuclear in the power mix to 50 percent by 2025 from the current 75 percent. But Hollande had taken no concrete steps towards closing any reactors.

Centrist Macron, elected in May, had promised to respect this target and the popular Hulot, one of France's best-known environmentalists, had said in July France might have to close as many as 17 of its 58 reactors by 2025 to achieve that target.

Widely seen as the guardian of the Macron government's green credentials, Hulot - a former television documentary maker turned environmentalist - had in recent months repeatedly said that France needs to cut its reliance on nuclear.

But state-owned utility EDF, which operates France's nuclear reactors, has long argued that it made no sense to shut down functioning reactors and instead wanted to extend the lifespan of its nuclear fleet from 40 to at least 50 years. (Reporting by Geert De Clercq and Michel Rose; Editing by Ingrid Melander)